3 Answers
3

Any line that begins with a # is a comment in many languages and is ignored by the interpreter (perl etc.).

However, if the first line of a script in Linux begins with a #! (shebang as it is called), it is not a comment but a directive to the program loader to actually run the program specified after #! and pass it the name of your file as the last argument.

For example, if the first line is

#!/usr/bin/perl -w

it means the shell will actually invoke /usr/bin/perl -w /path/to/the/script and you don't need to specify a program to run this script, you can run it using

/path/to/the/script

if you have the permission to run it and it is located on a filesystem supported for execution and the file has the permission to be executed.

For the interpreter, however, this line is always just a comment, so if the script is executed as:

perl /path/to/the/script

then the line has no effect. (Thanks to Ruslan for pointing this out).

Be warned that # is not always indicative of a comment. For example, a statement beginning with a # in C is pre-processor directive and not a comment.

In your case, the line is a comment and will be ignored while execution.

Update:

The file you are talking about is a menu.lst for which a comment is a line beginning with ## and not #. (Source)

# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#

This example is from the /etc/fstab file. The lines starting with # here are meant to help you how to correctly use the file. They do not have an actual function other than that.

in some files, more than one # is used before commented lines:

## comment line one.
## comment line two with more information

The first example of your question: #!/usr/bin/perl -w however, is a shebang, see this question for more about that subject.

These are the general rules for commenting. However in some (mostly language-specific) files, exceptions are possible to the general rules. In PHP for example, comments are made between /**/